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I made a video about the differences between a group 31 battery and a group 34 battery and when you should pick one over the other.

I couldnt figure out the main uses for each one when I upgraded and I couldnt find a video comparing the 2, so I thought that I could share the information I gathered while doing my own research. I hope this video helped someone out.

a few comments: not all batteries in a group have the same CCA or RC (for example my 31m has 800 CCA, also an old rule of thumb is lower CCA means the battery is more okay with deep cycling); not all group 31 batteries need battery terminal extensions (and you can make them for probably <$10 if you want); you don't need to a battery box for group 31, you could just get a basic battery tray for group 31 size batteries (or if you really want a box you can get them for a lot less than $200, here is the LFD tray for $70); the stock alternator can charge a group 31 no different than a group 34, it's just a matter of how much power you take out and how much you put in (both will need voltage boost for most AGMs)

also, don't forget Group 24 or Group 27. Group 24 is the same size as stock (no spacer needed) but has about 75Ah, Group 27 is a bit smaller than 31 and about 90Ah